Sorting PHP arrays with substrings

A couple of days back I encountered the following request on a forum for sorting a single dimensional array. The programmer wanted to sort the following array by the substring after the colon. For example ‘CDF’ in the string ‘66345:CDF’. Those values that do not have any colon should be ignored and pushed to the end of the array. Also, the length of the strings are not constant.

$values = array("66345:CDF", "61179:HGT", "64146:ABA",
                "68768:BNG", "68015:ZCZ", "80231:LPO",
                "64146:QWP", "68736:HHB", "86801:MNV",
                "80178:OIU", "80178:ASE", "88178:BRT",
                "801782OIU", "801378ASE", "881578BRT");

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Luhn algorithm for validating credit cards

The Luhn algorithm also known as the “modulus 10” or “mod 10” algorithm, is a checksum formula which can be used to validate credit card numbers. Developed in the 1950’s by IBM scientist Hans Peter Luhn and described in U.S. Patent 2,950,048. A PHP implementation is shown below.

function LuhnCheck($strDigits)
    $sum = 0;
    $alt = false;
    for($i = strlen($strDigits) - 1; $i >= 0; $i--) 
           $temp = $strDigits[$i];
           $temp *= 2;
           $strDigits[$i] = ($temp > 9) ? $temp = $temp - 9 : $temp;
        $sum += $strDigits[$i];
        $alt = !$alt;
    return $sum % 10 == 0;